The present invention relates to an electroacoustic transducer suitable for use in dynamic or electrostatic receiver or microphones that convert electrical signals to sound waves or vice versa.
Conventional dynamic receivers such as for use in telephone receivers employ a single diaphragm and a variety of methods have been proposed for realizing a broad flat frequency response using a single diaphragm. The construction of a typical dynamic receiver is shown in FIG. 11, wherein a casing generally indicated at 2 contains a first air chamber 6 in front of a diaphragm 4, as well as a coupler 10 that is disposed in front of the air chamber 6 with an intervening shield 8 having through holes 7 made in it. A coil 12 is disposed at the back of the diaphragm 4, and a cylindrical inner magnetic pole piece 14A surrounded by an annular outer magnetic pole piece 14B is also provided in the rear of the diaphragm 4. A second air chamber 16 is formed between the two magnetic pole pieces 14A and 14B. These magnetic pole pieces are attached to a wall plate 20 with an intervening paramagnetic plate 18 that forms a magnetic circuit together with the magnetic pole piece 14A. The wall plate 20 is provided with through holes 22 communicating with the second air chamber 16. At the back of the wall plate 20 is provided a third air chamber 26 that is coupled to the second air chamber 16 by the through holes 22.
An equivalent circuit of the dynamic receiver described above is shown in FIG. 12, wherein Sc stands for the stiffness of the coupler 10, S1 the stiffness of the first air chamber 6, S2 the stiffness of the second air chamber 16, S3 the stiffness of the third air chamber 26, S0 the stiffness of the diaphragm 4, Mo the mass (effective mass) of the diaphragm 4, M1 the mass of the through holes 7, m2 the mass of the through holes 22, r2 the damping resistance of the through holes 22, and F0 the driving source.
The principal elements of the dynamic receiver represented by the circuit of FIG. 12 that are associated with frequencies in the higher range are the stiffness S2 of the second air chamber 16, the mass m2 of the through holes 22 and the damping resistance r2 of the through holes 22. These elements are closely related to one another and it is very difficult to obtain the appropriate value of one element without being affected by another. As a result, the dynamic receiver has a frequency response typically shown in FIG. 13 wherein P1 and P2 represent peaks while D denotes a dip.
Such characteristics are highly deleterious to the quality of sound reproduced from the receiver. One of the approaches conventionally taken to avoid this problem is to provide an additional damping resistance by filling the through holes 22 with fiberglass. This method however is not suitable for mass production of receivers for several reasons such as non-uniformity in the characteristics of the products.
In addition to this difficulty in mass production, the adjustment of the damping resistance by the use of fiberglass causes other problems such as a complicated acoustic structure of the receiver and time- or environment-dependent changes of its frequency response.